LU-17197 obdclass: preserve fairness when waiting for rpc slot

LU-17197 obdclass: preserve fairness when waiting for rpc slot

When obd_get_mod_rpc_slot() waits for an available slot it places the
waiting thread at the HEAD of the queue, so it will be woken before
anything else that is already queued.  This is clearly unfair and can
hurt performance.

So change to always add to the tail to ensure a FIFO ordering (except
that CLOSE might sometimes be woken a bit early).

This regression was introduced in a rewrite that was supposed to make
waiting more fair - by avoiding a broadcast wakeup for "close"
requests.

Also fix some stale comments and expose __add_wait_queue_entry_tail

Running mdtest with the patch applied shows about a 3% improvement:

                             master            patched
  mdtest-easy-write      350.585906 kIOPS   353.783545 kIOPS
   mdtest-easy-stat     1320.329353 kIOPS  1408.320419 kIOPS
 mdtest-easy-delete      285.084103 kIOPS   289.625900 kIOPS
            [SCORE]      509.115803 kiops   524.516113 kiops

Fixes: 5243630b09d2 ("LU-15947 obdclass: improve precision of wakeups for mod_rpcs")
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: If767c4299bcbab71589b0f3c01e85bf461686ca5